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Tractive Force.


double spring-loaded safety valve, 31, is placed over the firebox.
The valves are inverted cones, fitting easily, and either centre-
point can be lifted by the lever 32 to test the working. A safety
link placed within the spring holds the lever in case of breakage.
71 is the steam whistle; 70, a lamp bracket; and 72 the chimney,
of cast iron. 38 are lubricators for the steam chests.

Tractive Force of a Locomotive is usually taken as
the mean pull exerted on the moving train, and may be estimated
from the principle of work. Thus :

Work given by Steam = Work done on Train.

Total mean pressure )
in both cylinders    J

x stroke = Tract, force x

/ half wheel
1 circumference

2 x "

/ = T x TT r

.-.   T =

The tractive force for any particular starting position can
only be found by first ascertaining the crank effort, for that
position (E) ; then, by moments :

2 r

Of course, the greatest value of T must not overcome the
adhesive force, or slipping will occur (seep. 571). The tractive
force required is given at p. 569.

Boiler Fittings.- Boilers having been described at pp. 330
to 339, it remains to consider the principal mountings with which
they are fitted, (See Appendix I^p. 755; Appendix //.,/. 833
and pp. 899-905; Appendix III., p. 918.) (See also p. 1148.)

Safety Valves. Lever-loaded valves, p. 482, are not now
in favour, on account of the fear of explosion due to sticking.
Directly-loaded valves may be either spring or weight loaded.
The former has been shewn at 31, Fig. 675, Plate XVIII ; and a
dead-weight valve is given at Fig. 678, as applied to stationary
boilers. A casing A, containing the weights, is hung on a cup-
shaped valve resting on the conical end of the pipe B. The
figure shews also a low-water float c and a high-water float D,
which raise rod E whenever the water falls too low or rises too
high respectively. Marine valves are spring-loaded, and the